The invention relates to silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs).
It is generally known to form HBTs by using wafers that include one or more layers of silicon germanium (SiGe) on a silicon substrate. On such substrates, the germanium atoms create mechanical strain in the composite film due to the difference in lattice constant between the SiGe film and the silicon substrate. In the plane of the silicon substrate the larger lattice constant of the SiGe lattice is compressed onto the smaller lattice constant of the silicon substrate. In the plane perpendicular to the the silicon substrate, the SiGe layer lattice constant is greater than that of the silicon substrate and thus is under tensile stress. This strain together with the Ge atom itself, creates a bandgap offset between the SiGe film and the underlaying native Si substrate. This bandgap offset provides the unique advantages of the SiGe HBT by creating a grading field in the base to enhance carrier diffusion across the base and thus improve transistor speed. SiGe HBTs have been used as transistors for small signal amplifiers (i.e. switching approximately 5 volts or less) to provide the switching speeds necessary for current wireless communications devices.
SiGe enhances charge mobility by introducing mechanical strain due to the lattice mismatches inherent in the Sixe2x80x94Ge compound; if there is too much Ge, or if the SiGe layer is too thick, the accepted wisdom in the art is that the resulting crystal dislocations will reduce both performance and yield. The performance penalty would be due to dislocations relieving the mechanical stresses that create the bandgap offsets that SiGe provides. The yield penalty would be due to the defects disturbing the crystallography of the substrate. In fact, this general understanding has become so widespread that it is generally acknowledged as the xe2x80x9cMatthews-Blakesley stability criterionxe2x80x9d or the xe2x80x9cStiffler limit,xe2x80x9d in recognition of the researchers who first reported these interrelationships (Stiffler et al., Journal of Applied Physics, Vol. 71, No. 10, pp. 4820-4825 (1994); Matthews and Blakeslee, xe2x80x9cDefects in Epitaxial Multilayers,xe2x80x9d Journal of Crystal Growth 27 pp. 118-125 (1974)). For ease of future reference, these results will be referred to as the xe2x80x9cSiGe stability limits.xe2x80x9d
The introduction of carbon (C) or other intert impurty atoms into the SiGebase region via low temperature epitaxy (LTE) growth is an effective way to restrict base boron (B) outdiffusion. Typically, both boron and carbon are introduced into the ambient during the LTE process, at different times and at different concentrations. See for example xe2x80x9cSuppression of boron outdiffusion in SiGe HBTs by carbon incorporation,xe2x80x9d Lanzerotti, L. D.; Sturm, J. C.; Stach, E.; Hull, R.; Buyuklimanli, T.; Magee, C. This paper appears in: Electron Devices Meeting 1996, International, page(s): 249-252, Dec. 8-11, 1996.
In order to enhance the performance of the SiGe HBT, it is preferred to increase the concentration of Ge to above 20% or so. This increases the mobility advantages of SiGe substrates, and is required for applications with an Ft greater than approximately 100 Ghz. However, the inventors found that HBTs built with this high Ge concentration in the presence of a boron-diffusion-limiting impurity (eg. Carbon) suffered from yield issues. Thus, a need has developed in the art for a high performance SiGe HBT with a boron-doped base region and a boron-diffusion-limiting impurity region which avoids yield deterrents.
It is thus an object of the present invention to provide a high performance SiGe HBT with a boron-doped base region and a boron-diffusion-limiting impurity region.
It is another object of the invention to provide a high performance SiGe HBT that does not suffer from performance or yield problems.
The foregoing and other objects of the invention are realized, in a first aspect, by a high performance SiGe HBT that has a SiGe layer with a peak Ge concentration of at least approximately 20% and a boron-doped base region formed therein having a thickness, wherein said base region includes diffusion-limiting impurities throughout said thickness at a concentration below that of boron in said base region, and wherein said diffusion limiting impurities are physically located relative to both said base region and a portion of said SiGe layer having a relatively high concentration of Ge to optimize performance and yield of said SiGe HBT
Another aspect of the invention is a high performance SiGe HBT that has a SiGe layer with a peak Ge concentration of at least approximately 20%, a boron-doped base region formed therein having a thickness, and a region of diffusion-limiting impurities at a concentration, thickness, and spacing relative to said base region and a portion of said SiGe layer having a peak concentration of Ge that optimizes both performance and yield of said SiGe HBT.
A further aspect of the invention is a method for forming a high performance SiGe layer on a Si substrate, comprising the steps of introducing germanium atoms during formation of a Si layer; introducing diffusion-limiting impurities and boron atoms during formation of said Si layer, while said germanium atoms are still being introduced; and terminating both said diffusion-limiting impurities and said boron atoms approximately simultaneously, said diffusion limiting impurities being introduced at a concentration and for a duration that optimizes both performance and yield.